U.S. patent application number 15/517399 was filed with the patent office on 2017-11-30 for graphene powder, electrode paste for lithium ion battery and electrode for lithium ion battery.
This patent application is currently assigned to TORAY INDUSTRIES, INC.. The applicant listed for this patent is TORAY INDUSTRIES, INC.. Invention is credited to Yasuo Kubota, Eiichiro Tamaki, Hanxiao Yang.
Application Number | 20170346098 15/517399 |
Document ID | / |
Family ID | 55653168 |
Filed Date | 2017-11-30 |
United States Patent
Application |
20170346098 |
Kind Code |
A1 |
Yang; Hanxiao ; et
al. |
November 30, 2017 |
GRAPHENE POWDER, ELECTRODE PASTE FOR LITHIUM ION BATTERY AND
ELECTRODE FOR LITHIUM ION BATTERY
Abstract
The present invention relates to preparation of a highly
dispersible graphene powder. Further, the present invention
includes providing an electrode for a lithium ion battery having
good output characteristics and cycle characteristics by utilizing
a highly dispersible graphene powder. The present invention also
includes providing a graphene powder having a specific surface area
of 80 m.sup.2/g or more to 250 m.sup.2/g or less as measured by BET
measurement, and an oxygen-to-carbon element ratio of 0.09 or more
to 0.30 or less as measured by X-ray photoelectron
spectroscopy.
Inventors: |
Yang; Hanxiao;
(Otsu-shi,Shiga, JP) ; Tamaki; Eiichiro;
(Otsu-shi,Shiga, JP) ; Kubota; Yasuo;
(Otsu-shi,Shiga, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TORAY INDUSTRIES, INC. |
CHUO-KU |
|
JP |
|
|
Assignee: |
TORAY INDUSTRIES, INC.
CHUO-KU
JP
|
Family ID: |
55653168 |
Appl. No.: |
15/517399 |
Filed: |
October 6, 2015 |
PCT Filed: |
October 6, 2015 |
PCT NO: |
PCT/JP2015/078362 |
371 Date: |
April 6, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C01B 2204/32 20130101;
Y02P 70/50 20151101; H01M 10/0525 20130101; Y10S 977/948 20130101;
H01M 4/625 20130101; Y10S 977/734 20130101; H01M 10/052 20130101;
Y10S 977/842 20130101; C01B 2204/30 20130101; B82Y 30/00 20130101;
Y02E 60/10 20130101; C01B 2204/22 20130101; C01B 32/192 20170801;
B82Y 40/00 20130101; C01P 2002/82 20130101; C01B 32/05
20170801 |
International
Class: |
H01M 4/62 20060101
H01M004/62; C01B 32/192 20060101 C01B032/192; H01M 10/0525 20100101
H01M010/0525 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 10, 2014 |
JP |
2014-208590 |
Claims
1. A graphene powder, having a specific surface area of 80
m.sup.2/g or more to 250 m.sup.2/g or less as measured by BET
measurement, and an oxygen-to-carbon element ratio (O/C ratio) of
0.09 or more to 0.30 or less as measured by X-ray photoelectron
spectroscopy.
2. The graphene powder according to claim 1, wherein a
nitrogen-to-carbon element ratio (N/C ratio) as measured by X-ray
photoelectron spectroscopy is 0.005 or more to 0.02 or less.
3. The graphene powder according to claim 1, wherein a ratio of
peak intensity I.sub.D to peak intensity I.sub.G (I.sub.D/I.sub.G
ratio) as measured by Raman spectroscopy is 1 or more to 2 or
less.
4. The graphene powder according to claim 1, having a powder
resistivity of 10.sup.-3 .OMEGA.cm or more to 10.sup.-1 .OMEGA.cm
or less.
5. An electrode paste for a lithium ion battery, comprising: the
graphene powder according to claim 1, an electrode active material,
and a binder.
6. An electrode for a lithium ion battery, comprising the graphene
powder according to claim 1.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is the U.S. National Phase application of
PCT/JP2015/078362, filed Oct. 6, 2015, and claims priority to
Japanese Patent Application No. 2014-208590, filed Oct. 10, 2014,
the disclosures of each of these applications being incorporated
herein by reference in their entireties for all purposes.
FIELD OF THE INVENTION
[0002] The present invention relates to a graphene powder, and an
electrode paste for a lithium ion battery containing the graphene
powder and an electrode for a lithium ion battery containing the
graphene powder.
BACKGROUND OF THE INVENTION
[0003] Graphene is a two-dimensional crystal composed of carbon
atoms, and is a material that has been greatly attracting attention
since being discovered in 2004. Graphene has excellent electrical,
thermal, optical, and mechanical properties, and is expected to be
applied in a wide range of battery materials, energy storage
materials, electronic devices, composite materials, and the
like.
[0004] In order to realize such an application of graphene, the
efficiency in a preparation method for cost reduction, and the
improvement of the dispersibility are essential issues.
[0005] As the production method of graphene, a mechanical
exfoliation method, a chemical vapor deposition (CVD) method, a
crystal epitaxial growth (CEG) method, and the like can be
mentioned, but these methods have low productivity and are not
suitable for mass production. On the other hand, an
oxidation-reduction method (in which graphite oxide or graphene
oxide is obtained by an oxidation treatment of natural graphite,
and then graphene is prepared by a reduction reaction) can
synthesize the graphene in a large amount, and is an extremely
important technique for putting the graphene into practical
use.
[0006] The graphene obtained as described above has high conductive
performance and further has a thin flaky structure, and therefore,
can increase the conductive path, and has high potential in
particular as a conductive material for a battery. However,
graphene is nanocarbon, and is easily aggregated. Even when
prepared simply by an oxidation-reduction method, the graphene
cannot be adequately dispersed and cannot exert the potential
either.
[0007] Accordingly, in Patent Document 1, graphite oxide is
expanded and exfoliated by heating, and flake graphite having a
high specific surface area is prepared. In Patent Document 2,
graphene oxide and an electrode active material for a lithium ion
battery are mixed, and then the mixture is reduced by heating, and
the resultant product is utilized as a conductive agent. In
addition, in Patent Document 3, graphene is reduced in the presence
of catechol, and highly dispersible graphene is prepared.
PATENT DOCUMENTS
[0008] Patent Document 1: Japanese Translation of PCT Application
No. 2009-511415
[0009] Patent Document 2: Japanese Patent Laid-open Publication No.
2014-112540
[0010] Patent Document 3: WO 2013/181994 A
SUMMARY OF THE INVENTION
[0011] However, as in Patent Document 1, graphene prepared by
thermal expansion has an excessively high specific surface area,
and induces the aggregation, and therefore, the graphene cannot be
favorably dispersed.
[0012] As in Patent Document 2, also in the technique of mixing
graphene oxide with other particles and heating the mixture, as in
the case of Patent Document 1, graphene is prepared by a heat
treatment, and therefore the specific surface area is increased.
Further, the oxygen ratio is decreased by the heating and the
dispersibility is also decreased.
[0013] In addition, when a surface treatment agent is used as in
Patent Document 3, although the dispersibility is increased,
particles of the graphene oxide are stacked on each other, and the
exfoliation state of a graphene powder after reduction becomes
insufficient.
[0014] As a result of keen study, the present inventors have found
that the graphene having both of an adequate specific surface area
and an adequate oxidation degree can have high dispersibility and
high ion conductivity with the thin shape.
[0015] That is, the present invention includes providing a graphene
powder having a specific surface area of 80 m.sup.2/g or more to
250 m.sup.2/g or less as measured by BET measurement, and an
oxygen-to-carbon element ratio of 0.09 or more to 0.30 or less as
measured by X-ray photoelectron spectroscopy.
[0016] The graphene powder of the present invention has both of an
adequate specific surface area and an adequate oxidation degree,
and therefore has high dispersibility and high ion conductivity. As
to a conductive agent, as the number per weight is larger and the
dispersibility is higher, the conductive network can be more easily
formed in a resin or in an electrode, and therefore the performance
is higher. Accordingly, by forming a conductive network in an
electrode matrix with the use of the graphene of the present
invention together with a binder and an electrode active material,
an electrode for a lithium ion battery having excellent discharge
performance can be provided.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0017] <Graphene Powder>
[0018] A graphene powder has a structure in which single layer
graphene is laminated, and has a flaky form. The thickness of the
graphene is not particularly limited, but is preferably 100 nm or
less, more preferably 50 nm or less, and further preferably 20 nm
or less. The size in a surface direction of the graphene is not
particularly limited either, but is preferably 0.5 .mu.m or more,
more preferably 0.7 .mu.m or more, and further preferably 1 .mu.m
or more as the lower limit, and is preferably 50 .mu.m or less,
more preferably 10 .mu.m or less, and further preferably 5 .mu.m or
less as the upper limit. The size in a surface direction of the
graphene as referred to herein means the average of the longest
diameter and the shortest diameter of the graphene surface.
Further, a surface treatment agent described later may be contained
in the graphene powder.
[0019] The specific surface area of the graphene powder of an
embodiment of the present invention as measured by BET measurement
(hereinafter also simply referred to as "specific surface area") is
80 m.sup.2/g or more to 250 m.sup.2/g or less. The specific surface
area of the graphene reflects the thickness and the exfoliation
degree of the graphene. As the specific surface area is larger, the
graphene is thinner and has higher exfoliation degree. When the
specific surface area of the graphene is less than 80 m.sup.2/g,
the number of graphene particles per unit weight is small in a case
where the graphene is mixed in an electrode or in a resin, and
therefore, the conductive network is hardly formed. When the
specific surface area of the graphene is larger than 250 m.sup.2/g,
particles of the graphene are easily aggregated each other, and the
aggregate exists isolatedly in an electrode or in a resin, and
therefore, the conductive network is hardly formed. The specific
surface area of the graphene is preferably 100 m.sup.2/g or more,
and more preferably 130 m.sup.2/g or more. Further, the specific
surface area of the graphene is preferably 200 m.sup.2/g or less,
and more preferably 180 m.sup.2/g or less. The BET measurement is
performed in accordance with a method described in JIS Z8830: 2013.
The measurement of the adsorption gas amount is measured by a
carrier gas method, and the analysis of the adsorption data is
performed by a one-point method.
[0020] The graphene powder in an embodiment of the present
invention has an oxygen-to-carbon element ratio (O/C ratio) of 0.09
or more to 0.30 or less. Oxygen atoms in the graphene powder are
the oxygen atoms contained in an acidic group bound to the graphene
itself or contained in an acidic group that exists in a surface
treatment agent adhered onto a surface of the graphene. Herein, the
acidic group means a hydroxy group, a phenolic hydroxy group, a
nitro group, a carboxyl group, or a carbonyl group, and these
groups have an effect of improving the dispersion state of the
graphene. When there are extremely few oxygen atoms in a graphene
powder, the dispersibility of the graphene powder in a case of
mixing the graphene powder into an electrode or into a resin is
poor. Therefore, the O/C ratio is preferably 0.10 or more. Further,
when there are extremely many oxygen atoms in a graphene powder,
the graphene is in a state of not being sufficiently reduced, and
the electrical conductivity is decreased. Therefore, the O/C ratio
is preferably 0.20 or less, and more preferably 0.15 or less.
[0021] In the present invention, the O/C ratio is a value
determined from the amounts of carbon atoms and oxygen atoms which
are measured by X-ray photoelectron spectroscopy. In the X-ray
photoelectron spectroscopy, a surface of a sample placed in
ultrahigh vacuum is irradiated with soft X-rays, and the
photoelectrons emitted from the surface of the sample are detected
by an analyzer. By measuring the photoelectrons with wide scanning,
and determining the bond energy value of bound electrons in a
substance, elemental data of the substance can be obtained.
Further, the O/C ratio of the graphene powder can be determined
from peak areas of carbon atoms and oxygen atoms.
[0022] The O/C ratio can be controlled by changing the oxidation
degree of the graphene oxide as a raw material, or by changing the
amount of a surface treatment agent. The higher the oxidation
degree of the graphene oxide is, the larger the amount of the
remaining oxygen atoms after reduction is. When the oxidation
degree is low, the amount of oxygen atoms after reduction is
reduced. Further, by increasing the adhesion amount of a surface
treatment agent that has an acidic group, the amount of oxygen
atoms can be increased.
[0023] When the graphene powder of the present invention is in the
range of specific surface area described above and the range of O/C
ratio described above, the graphene powder has high exfoliation
degree, and further has favorable dispersibility in an electrode or
in a resin, and can form an ideal conductive network. In the
graphene powder of the present invention, a nitrogen-to-carbon
element ratio (N/C ratio) is preferably 0.005 or more to 0.02 or
less. Nitrogen atoms in the graphene powder are the nitrogen atoms
derived from a nitrogen-containing functional group such as an
amino group and a nitro group contained in a surface treatment
agent, or a heterocyclic compound containing nitrogen of a pyridine
group or an imidazole group. When the element composition ratio of
nitrogen atoms to carbon atoms exceeds 0.02, the nitrogen atoms
replace the graphene conjugated structure, and therefore, the
electrical conductivity is easily lowered. On the other hand, the
surface treatment agent containing a nitrogen element particularly
contributes to the graphene dispersibility in a solvent, and
therefore is preferably present in a small amount. From the
viewpoint described above, the N/C ratio is further preferably 0.01
or more to 0.015 or less. The N/C ratio is a value measured by
X-ray photoelectron spectroscopy similarly to the O/C ratio.
[0024] It is preferred that the graphene powder of the present
invention has some degree of structural defects from the point of
improving the ion conductivity. When the graphene powder contains
structural defects, ions can move through the structural defects,
and therefore, the ion conductivity can be improved. When there are
extremely few structural defects, ions cannot pass through the
graphene layer in a direction perpendicular to the layer, and
therefore, the ion conductivity is lowered. Further, when there are
extremely many structural defects, the electrical conductivity is
lowered.
[0025] The structural defects of the graphene powder can be
measured by Raman spectroscopy. In a perfect graphite crystal,
intrinsically a peak of I.sub.D does not appear, but as the
symmetry of the graphite structure is lost, the peak intensity of
I.sub.D is increased. Accordingly, as the structural defects of the
graphene powder increase, the peak intensity ratio of
I.sub.D/I.sub.G(I.sub.D/I.sub.G ratio) decreases. From the
viewpoint of achieving a balance between the ion conductivity and
the electrical conductivity, the I.sub.D/I.sub.G ratio is
preferably 1 or more to 2 or less, more preferably 1.3 or more to
1.8 or less, and further preferably 1.45 or more to 1.7 or
less.
[0026] In addition, the peak intensity ratios of Raman measurement
are all obtained by the measurement at an excitation wavelength of
514.5 nm using an argon ion laser as an excitation laser. In the
Raman spectroscopy, the graphene powder has peaks in the vicinity
of 1580 cm.sup.-1 and in the vicinity of 1335 cm.sup.-1. The peak
intensity in the vicinity of 1580 cm.sup.-1 is designated as
I.sub.G, and the peak intensity in the vicinity of 1335 cm.sup.-1
is designated as I.sub.D.
[0027] The powder resistivity of the graphene powder in the present
invention is preferably 10.sup.-3 .OMEGA.cm or more to 10.sup.-1
.OMEGA.cm or less, and more preferably 1.times.10.sup.-3 .OMEGA.cm
or more to 3.times.10.sup.-2 .OMEGA.cm or less. The powder
resistivity correlates with the adhesion amount of a surface
treatment agent. Accordingly, when the powder resistivity is less
than 10.sup.-3 .OMEGA.cm, the adhesion amount of a surface
treatment agent is insufficient and the dispersibility tends to be
decreased. On the other hand, when the powder resistivity exceeds
10.sup.-1 .OMEGA.cm, there is a tendency that the electrical
conductivity is lowered and the performance as a conductive agent
is deteriorated.
[0028] <Electrode for Lithium Ion Battery>
[0029] The electrode for a lithium ion battery of an embodiment of
the present invention contains a positive or negative electrode
active material, and the graphene powder of the present invention
as a conductive agent, and is typically an electrode in which a
mixture layer containing an electrode active material, the graphene
powder of the present invention, and a binder is formed on a
collector.
[0030] The type of the collector is not limited as long as it is a
sheet or mesh having electrical conductivity, and a collector of
metal foil or metal mesh, which does not largely affect the
electrochemical reaction, is used. As the collector on the positive
side, a collector of aluminum foil or aluminum mesh is preferred.
As the collector on the negative side, a collector of copper foil
or copper mesh is preferred. In order to increase the electrode
density, there may be pores in part of the metal foil.
[0031] The electrode active material is roughly classified into a
positive electrode active material and a negative electrode active
material. The graphene powder of the present invention can be
utilized for either of the positive electrode active material and
the negative electrode active material. The positive electrode
active material is not particularly limited, and examples of the
positive electrode active material include composite oxides of
lithium and a transition metal, such as lithium cobaltate
(LiCoO.sub.2), lithium nickelate (LiNiO.sub.2), spinel-type lithium
manganate (LiMn.sub.2O.sub.4), or a ternary system material in
which a portion of cobalt is substituted with nickel and manganese
(LiMn.sub.xNi.sub.yCo.sub.1-x-yO.sub.2), and spinel-type lithium
manganate (LiMn.sub.2O.sub.4), olivine-based (phosphate-based)
active materials such as lithium iron phosphate (LiFePO.sub.4),
metal oxides such as V.sub.2O.sub.5, metal compounds such as
TiS.sub.2, MoS.sub.2 and NbSe.sub.2, elemental sulfur, and organic
positive electrode materials. The negative electrode active
material is not particularly limited, and examples of the negative
electrode active material include carbon materials such as natural
graphite, artificial graphite, and hard carbon; silicon compounds
in which SiO, SiC, SiOC or the like is contained as a basic
constituent element; elemental silicon; and metal oxides such as
manganese oxide (MnO) and cobalt oxide (CoO), which can be reactive
with a lithium ion in a conversion manner.
[0032] As the conductive agent, only the graphene powder of the
present invention may be used, or another conductive agent may
further be added. The conductive agent to be further added is not
particularly limited, and examples thereof include carbon blacks
such as furnace black, ketjen black, and acetylene black; graphites
such as natural graphite (scaly graphite and the like), and
artificial graphite; conductive fibers such as carbon fibers and
metal fibers; and metal powders of copper, nickel, aluminum,
silver, or the like.
[0033] As the binder, a fluoropolymer such as polyvinylidene
fluoride (PVDF) and polytetrafluoroethylene (PTFE), or a rubber
such as styrene-butadiene rubber (SBR) and natural rubber can be
used.
[0034] By mixing these active materials, a conductive agent, and a
binder as needed with a solvent in an adequate amount, an electrode
paste for a lithium ion battery can be prepared. In addition, by
applying the electrode paste to a collector and drying the
electrode paste, an electrode for a lithium ion battery can be
prepared. As the solvent used herein, N-methyl pyrrolidone,
.gamma.-butyrolactone, carboxymethyl cellulose, dimethylacetamide,
or the like may be used, and N-methyl pyrrolidone is particularly
preferably used.
[0035] The technique of mixing an electrode paste for a lithium ion
battery is not limited, and a known mixer/kneader can be used.
Examples of the known mixer include an automatic mortar, a
homogenizer, a planetary mixer, a homodisper, and a
rotation-revolution mixer. A planetary mixer can be mentioned as a
particularly preferred technique.
[0036] In addition, by applying the electrode paste to a collector
and drying the electrode paste, an electrode for a lithium ion
battery can be prepared. The method for applying the electrode
paste to a collector is not particularly limited, and the electrode
paste can be applied by using a baker-type applicator, a film
applicator with a micrometer, a bar coater, a doctor blade, or the
like manually or with an automatic coating machine.
[0037] The graphene powder of the present invention has a specific
surface area and an oxidation degree each in a specific range, and
therefore can be favorably dispersed in an electrode paste solvent.
Accordingly, the electrode for a lithium ion battery of the present
invention can improve the electron conductivity in the electrode
because the graphene powder favorably disperses in the electrode,
and an electrode for a lithium ion battery having excellent
performance can be provided.
[0038] <Production Method of Graphene Powder>
[0039] The graphene powder of the present invention can be
prepared, as an example, by a production method in which graphene
oxide, and a surface treatment agent having an acidic group are
mixed in a solvent, and then the graphene oxide is subjected to a
reduction treatment. Particles of the graphene having an adequate
oxidation degree and containing functional groups in a large amount
easily interact with each other, and there is a tendency that the
specific surface area is lowered because of the overlapping of the
graphene. In particular, a graphene powder having an
oxygen-to-carbon element ratio of 0.09 or more to 0.30 or less and
a nitrogen-to-carbon element ratio of 0.005 or more to 0.02 or less
has a tendency to lower the specific surface area. Accordingly, in
order to increase the specific surface area of the graphene having
an adequate oxygen ratio and an adequate nitrogen ratio, it is
required to perform an exfoliation treatment by shearing, to select
an appropriate reduction technique, and to select an appropriate
drying technique.
[0040] The preparation method of the graphene oxide is not
particularly limited, and a known method such as a Hummers' method
can be used. Further, graphene oxide available on the market may be
purchased. As the preparation method of the graphene oxide, a
method in a case of using a Hummers' method is mentioned in the
following.
[0041] Graphite (black lead powder) and sodium nitrate are put into
concentrated sulfuric acid, and into the resultant mixture,
potassium permanganate is gradually added with stirring so that the
temperature is not raised, and the resultant mixture is stirred and
reacted at a temperature of 25 to 50.degree. C. for 0.2 to 5 hours.
After that, ion-exchange water is added into the resultant mixture
to dilute the mixture, whereby a suspension is obtained. Then, the
suspension is reacted at a temperature of 80 to 100.degree. C. for
5 to 50 minutes. Finally, hydrogen peroxide and deionized water are
added, and the resultant mixture is reacted for 1 to 30 minutes to
give a graphene oxide dispersion. The obtained graphene oxide
dispersion is filtrated and washed to give a graphene oxide gel.
The graphene oxide gel may be diluted, and then mixed with a
surface treatment agent, or subjected to a reduction treatment.
Alternatively, by removing the solvent from the graphene oxide gel
by freeze drying, spray drying, or the like, a graphene oxide
powder is obtained, and then the graphene oxide powder may be
dispersed in a solvent and subjected to a treatment. However, when
the graphene oxide is dried, particles of the graphene oxide are
stacked on each other, and the specific surface area is easily
lowered. Therefore, it is preferred that the graphene is prepared
without passing through a step of drying the graphene oxide.
[0042] The graphite as a raw material of the graphene oxide may be
either of artificial graphite and natural graphite, but natural
graphite is preferably used. The number of mesh of the graphite as
a raw material is preferably 20000 or less, and more preferably
5000 or less.
[0043] As an example, the proportion of the reactants is 150 to 300
ml of concentrated sulfuric acid, 2 to 8 g of sodium nitrate, 10 to
40 g of potassium permanganate, and 40 to 80 g of hydrogen
peroxide, relative to 10 g of graphite. When sodium nitrate and
potassium permanganate are added, the temperature is controlled by
utilizing an ice bath. When hydrogen peroxide and deionized water
are added, the mass of the deionized water is 10 to 20 times as
much as the mass of hydrogen peroxide. As concentrated sulfuric
acid, concentrated sulfuric acid having a mass content of 70% or
more is preferably used, and concentrated sulfuric acid having a
mass content of 97% or more is more preferably used.
[0044] The graphene oxide has high dispersibility, but the graphene
oxide itself is insulative and cannot be used as a conductive agent
or the like. When the oxidation degree of the graphene oxide is
extremely high, there may be a case where the electrical
conductivity of the graphene powder obtained by reduction is
deteriorated. Therefore, the proportion of the carbon atoms to the
oxygen atoms in the graphene oxide, which is measured by X-ray
photoelectron spectroscopy, is preferably 0.5 or less. At the time
of measuring the graphene oxide by X-ray photoelectron
spectroscopy, the measurement is performed in a state where the
solvent has been sufficiently removed.
[0045] In addition, in a case where the graphite is not oxidized to
the inside, the graphene powder in a flaky form is hardly obtained
when the graphene oxide is reduced. Accordingly, it is desirable
for the graphene oxide that a peak specific to a graphite structure
is not detected when the dried graphene oxide powder is measured by
X-ray diffraction.
[0046] The oxidation degree of the graphene oxide can be adjusted
by changing the amount of an oxidizing agent to be used for the
oxidation reaction of graphite. Specifically, as the amounts of
sodium nitrate and potassium permanganate, which are used in the
oxidation reaction, are larger relative to the amount of graphite,
the oxidation degree is higher, and as the amounts are smaller, the
oxidation degree is lower. The weight ratio of sodium nitrate to
graphite is not particularly limited, but is preferably 0.20 or
more to 0.80 or less, more preferably 0.25 or more to 0.50 or less,
and particularly preferably 0.275 or more to 0.425 or less. The
ratio of potassium permanganate to graphite is not particularly
limited, but is preferably 1.0 or more, more preferably 1.4 or
more, and particularly preferably 1.65 or more. Further, the ratio
of potassium permanganate to graphite is preferably 4.0 or less,
more preferably 3.0 or less, and particularly preferably 2.55 or
less.
[0047] Next, the graphene oxide is mixed with a surface treatment
agent having an acidic group, that is, a hydroxy group, a phenolic
hydroxy group, a nitro group, a carboxyl group, or a carbonyl group
(hereinafter simply referred to as a "surface treatment agent").
The surface treatment agent is not limited as long as it has an
acidic group, and a polymer having an acidic group, a surfactant,
and a low-molecular compound can be mentioned.
[0048] Examples of the polymer having an acidic group include
polyvinyl pyrrolidone, polyvinyl alcohol, and polymethyl vinyl
ether. As the surfactant, any of a cationic surfactant, an anionic
surfactant, a nonionic surfactant, or the like can be used. Since
an anion or a cation itself may be involved in an electrochemical
reaction, a nonionic surfactant that is not ionized is suitable
from the viewpoint of being used as a battery material. Further,
from the viewpoint of enhancing the electrical conductivity of the
graphene, a low-molecular compound is preferred as compared to a
compound having a high molecular weight, such as a polymer and a
surfactant. As the low-molecular compound, a compound having an
aromatic ring is preferred from the viewpoint of the affinity for a
surface of the graphene.
[0049] As the acidic group possessed by a surface treatment agent,
a phenolic hydroxy group is preferred. Examples of the compound
having a phenolic hydroxy group include phenol, nitrophenol,
cresol, catechol, and a compound having a structure in which a
portion of phenol, nitrophenol, cresol, or catechol is substituted.
Among them, a compound having a catechol group is preferred because
of having adhesiveness to graphene and dispersibility in a solvent.
The surface treatment agent may have a basic group in addition to
an acidic group, and when the surface treatment agent has, in
particular, an amino group, the dispersibility is further improved.
A compound having both of a catechol group and an amino group is
particularly preferred.
[0050] The graphene oxide and a surface treatment agent can be
mixed by adding the surface treatment agent into a graphene oxide
dispersion, and stirring the resultant mixture. In order to
favorably mix the graphene oxide with the surface treatment agent,
the graphene oxide and the surface treatment agent are preferably
in a state of being dispersed in a solution. In this case, it is
preferred that the graphene oxide and the surface treatment agent
having an acidic group are both completely dissolved, but part of
the graphene oxide and surface treatment agent may be left as a
solid without being dissolved. As the solvent, a polar solvent is
preferred. The solvent is not particularly limited, and examples of
the solvent include water, ethanol, methanol, 1-propanol,
2-propanol, N-methyl pyrrolidone, dimethylformamide,
dimethylacetamide, dimethyl sulfoxide, and .gamma.-butyrolactone.
By performing the reduction in a state where the surface treatment
agent and the graphene oxide are mixed, graphene having a surface
treatment agent adhered thereto can be prepared.
[0051] In the present invention, the method of a reduction
treatment for graphene oxide is not limited. In a case of reduction
by heating, carbon dioxide is desorbed from the graphene oxide at
the time of the reduction reaction, and therefore, there is a
tendency that carbon falls out of the graphene structure and the
electrical conductivity is lowered. Further, in a case where the
graphene oxide is subjected to heat reduction, the reduction
reaction rapidly occurs and exfoliation occurs, and therefore,
there is a tendency that the specific surface area becomes
extremely large. On the other hand, in the reduction by chemical
reduction, the graphene structure is hardly broken as compared to
the reduction by heating, and further the reduction reaction occurs
moderately. Therefore, the chemical reduction is preferred as the
reduction technique. Examples of the reducing agent for chemical
reduction include an organic reducing agent and an inorganic
reducing agent, and an inorganic reducing agent is preferred
because of ease of washing after the reduction.
[0052] Examples of the organic reducing agent include an
aldehyde-based reducing agent, a hydrazine derivative reducing
agent, and an alcohol reducing agent. Among them, an alcohol
reducing agent enables relatively gentle reduction, and therefore,
it is particularly suitable. Examples of the alcohol reducing agent
include methanol, ethanol, propanol, isopropyl alcohol, butanol,
benzyl alcohol, phenol, ethanol amine, ethylene glycol, propylene
glycol, and diethylene glycol.
[0053] Examples of the inorganic reducing agent include sodium
dithionite, potassium dithionite, phosphorous acid, sodium
borohydride, and hydrazine. Among them, sodium dithionite and
potassium dithionite are suitably used because they are capable of
performing the reduction while relatively retaining functional
groups.
[0054] The production method of the graphene powder of the present
invention preferably includes a step of dispersing a mixture of a
graphene oxide powder or graphene oxide and a surface treatment
agent in a dispersion medium, and performing a stirring treatment
by a high shear mixer (this step is referred to as a stirring step)
at any stage before the above-described reduction treatment. The
stirring step may be performed before the mixing of graphene oxide
and a surface treatment agent, or may be performed at the same time
as the mixing of graphene oxide and a surface treatment agent. That
is, graphene oxide and a surface treatment agent may also be mixed
by stirring with a high shear mixer. Further, the stirring step may
be performed anew after the mixing of graphene oxide and a surface
treatment agent. In the stirring step, by performing the
exfoliation of graphene oxide with a high shear mixer, the specific
surface area can be increased.
[0055] The dispersion medium in the stirring step is not
particularly limited, but a dispersion medium that dissolves the
graphene oxide partly or totally is preferably used. As such a
dispersion medium, a polar solvent is preferred, and preferable
examples of the dispersion medium include water, ethanol, methanol,
1-propanol, 2-propanol, N-methyl pyrrolidone, dimethylformamide,
dimethylacetamide, dimethyl sulfoxide, and .gamma.-butyrolactone.
Among them, water has extremely high affinity and solubility for
graphene oxide, and is the most preferable solvent.
[0056] The shear rate in the stirring step is 10000 per second to
30000 per second. When the shear rate is extremely low, the
exfoliation of graphene oxide hardly occurs, and the specific
surface area of the graphene powder finally purified is low. On the
other hand, when the shear rate is extremely high, the specific
surface area of the graphene powder is high. The shear rate is
preferably 13000 or more per second, and more preferably 16000 or
more per second. Further, the shear rate is preferably 27000 or
less per second, and more preferably 24000 or less per second.
Moreover, the treatment time of the stirring treatment is
preferably 15 seconds to 300 seconds, and more preferably 30
seconds to 60 seconds.
[0057] As the high shear mixer used in a stirring step, FILMIX
(Registered Trademark) 30-30 Type (manufactured by PRIMIX
Corporation) can be mentioned. This high shear mixer has a gap of
around 1 mm between the turning part and the wall surface, and is
capable of applying high shear force by turning the turning part at
high speed.
[0058] The graphene obtained by reduction is appropriately washed,
and then dried to give a graphene powder. The drying method is not
limited, but graphene is aggregated during the drying, and
therefore, there may be a case where the specific surface area is
lowered. Accordingly, as the drying method for graphene, vacuum
drying is preferred, and freeze drying is more preferred, as
compared to normal pressure heat drying.
EXAMPLES
Measurement Example 1: Measurement of BET Specific Surface Area
[0059] The specific surface area of each sample was measured by
using HM Model-1210 (manufactured by Macsorb). As a measurement
principle, the measurement was performed by a BET flow method
(one-point type, a method described in Z8830: 2013). The degassing
condition was set to 100.degree. C..times.180 minutes, and the
equilibrium relative pressure was set to 0.29.
Measurement Example 2: Measurement of X-Ray Photoelectron
[0060] The measurement of X-ray photoelectron of each sample was
performed by using Quantera SXM (manufactured by Physical
Electronics, Inc.). An excited X-ray was monochromatic Al K.alpha.1
and K.alpha.2 lines (1486.6 eV), the X-ray diameter was set to 200
.mu.m, and the photoelectron escape angle was set to
45.degree..
Measurement Example 3: Raman Measurement
[0061] The Raman measurement was performed by using Ramanor T-64000
(manufactured by Jobin Yvon S.A.S./Atago Bussan Co., Ltd.). The
beam diameter was set to 100 .mu.m, and an argon ion laser
(wavelength: 514.5 nm) was used as a light source.
Measurement Example 4: Measurement of Powder Resistivity
[0062] The electrical conductivity of each sample was measured by
forming the sample into a disk-shaped test piece having a diameter
of around 20 mm and a density of 1 g/cm.sup.3, and by using the
disk-shaped test piece, with a high resistivity meter: MCP-HT450
and a low resistivity meter: MCP-T610 which are manufactured by
Mitsubishi Chemical Corporation.
Measurement Example 5: Measurement of Viscosity Yield Value
[0063] The viscosity yield value was measured by using an electrode
paste obtained by mixing 1.5 parts by weight of the graphene powder
prepared in the following example, 92 parts by weight of
LiMn.sub.2O.sub.4 as an electrode active material, 1.5 parts by
weight of acetylene black as another conductive agent, 5 parts by
weight of polyvinylidene fluoride as a binder, and 100 parts by
weight of N-methyl pyrrolidone as a solvent with a planetary mixer.
The yield value of the electrode paste was measured by using a
viscometer (manufactured by RHEOTECH, Model number RC20). The
viscosity was measured using a cone plate (C25-2) as a probe in 30
stages at a shear rate of 0 to 500 per second in a temperature
condition of 25.degree. C. by increasing the shear rate in stages.
The shear rate and the shear stress were plotted by Casson plot,
and the yield value was calculated from the intercept.
Measurement Example 6: Battery Performance Evaluation
[0064] The discharge capacity was measured as follows except as
otherwise described. The electrode paste prepared by the method
described in Measurement Example 5 was applied to aluminum foil
(having a thickness of 18 .mu.m) by using a doctor blade (300
.mu.m), and subjected to drying at 80.degree. C. for 15 minutes.
Then, the aluminum foil with the electrode paste was subjected to
vacuum drying to give an electrode plate. By using a piece having a
diameter of 15.9 mm cut out from the prepared electrode plate as a
positive electrode, a piece having a diameter of 16.1 mm and a
thickness of 0.2 mm cut out from lithium foil as a negative
electrode, a piece having a diameter of 17 mm cut out from Celgard
#2400 (manufactured by Celgard, LLC.) as a separator, and a solvent
of ethylene carbonate:diethyl carbonate=7:3 containing 1 M of
LiPF.sub.6 as an electrolytic solution, a 2042-type coin battery
was prepared, and electrochemical evaluation was performed. The
charge and discharge measurement was performed 3 times each at a
rate of 0.1 C, 1 C, and 5 C in this order, 9 times in total, with
an upper limit voltage of 4.3 V and a lower limit voltage of 3.0 V,
and the capacity at the time of the third discharge at a rate of 5
C was defined as the discharge capacity.
Synthesis Example 1
[0065] Preparation method of graphene oxide: A 1500 mesh natural
graphite powder (manufactured by Shanghai Yifan Graphite Co., Ltd.)
was used as a raw material. In 10 g of the natural graphite powder
in an ice bath, 220 ml of 98% concentrated sulfuric acid, 5 g of
sodium nitrate, and 30 g of potassium permanganate were added. The
resultant mixture was mechanically stirred for 1 hour, and the
temperature of the mixture was kept at 20.degree. C. or lower. This
mixture was taken out of the ice bath, and stirred for 4 hours in a
water bath at 35.degree. C. to be reacted, and then 500 ml of
ion-exchange water was added to the resultant mixture to give a
suspension. The suspension was further reacted at 90.degree. C. for
15 minutes. Finally, 600 ml of ion-exchange water and 50 ml of
hydrogen peroxide were put into the suspension, and the mixture was
reacted for 5 minutes to give a graphene oxide dispersion. The
graphene oxide dispersion was filtered while it was hot, and the
metal ions were washed with a dilute hydrochloric acid solution.
The acid was washed with ion-exchange water, and the washing was
repeated until the pH became 7 to prepare a graphene oxide gel. The
element composition ratio of oxygen atoms to carbon atoms (O/C
ratio) in the prepared graphene oxide gel was 0.53.
Example 1
[0066] (1) Preparation method of graphene powder: The graphene
oxide gel prepared in Synthesis Example 1 was diluted with
ion-exchange water to a concentration of 30 mg/ml, and treated for
30 minutes with an ultrasonic washing machine to give a homogenized
graphene oxide dispersion.
[0067] The graphene oxide dispersion in a volume of 20 ml was mixed
with 0.3 g of dopamine hydrochloride, and the mixture was treated
at a rotational speed of 40 m/s (shear rate: 20000 per second) for
60 seconds with FILMIX (Registered Trademark) 30-30 Type
(manufactured by PRIMIX Corporation). After the treatment, the
graphene oxide dispersion was diluted to a concentration of 5
mg/ml, and 0.3 g of sodium dithionite was put into 20 ml of the
dispersion. The resultant mixture was reacted at a reduction
reaction temperature of room temperature (40.degree. C.) for a
reduction reaction time of 1 hour, filtered, water-washed, and
freeze-dried to give a graphene powder.
[0068] (2) Properties and Performance of Graphene Powder
[0069] The specific surface area of the prepared graphene powder
was 180 m.sup.2/g as measured in accordance with the procedures in
Measurement Example 1. When the measurement of X-ray photoelectron
was performed in accordance with the procedures in Measurement
Example 2, the element composition ratio of oxygen atoms to carbon
atoms (O/C ratio) was 0.12, and the element composition ratio of
nitrogen atoms to carbon atoms (N/C ratio) was 0.013. When the
prepared graphene powder was measured by Raman spectroscopy in
accordance with the procedures in Measurement Example 3, the
I.sub.D/I.sub.G ratio was 1.55. The powder resistivity was
4.2.times.10.sup.-2 .OMEGA.cm as measured in accordance with the
procedures in Measurement Example 4.
[0070] Further, the viscosity yield value of the electrode paste
was 10 Pa as measured in accordance with the procedures in
Measurement Example 5. When the battery performance evaluation was
performed by using the paste in accordance with the procedures in
Measurement Example 6, the discharge capacity was 90 mAh/g.
Examples 2 to 4
[0071] The conditions of FILMIX at the time of stirring and/or the
concentration of graphene oxide were changed to those described in
Table 1.
Example 5
[0072] The treatment was performed in the same manner as in Example
1 except that the reducing agent was changed to 0.3 g of hydrazine
monohydrate.
Example 6
[0073] The treatment was performed in the same manner as in Example
1 except that the reducing agent was changed to 0.3 g of sodium
borohydride, and the reduction reaction temperature was changed to
60.degree. C.
Example 7
[0074] The treatment was performed in the same manner as in Example
1 except that the drying method of graphene powder was changed from
freeze drying to vacuum drying at 80.degree. C. for 6 hours.
Comparative Example 1
[0075] The graphene oxide gel prepared in Synthesis Example 1 was
dried at 100.degree. C. for 24 hours in a heating furnace. The
dried graphene oxide was placed in a quartz tube filled with an
argon atmosphere, the quartz tube was rapidly placed in an electric
furnace preheated at 1050.degree. C., and kept for 30 seconds in
the furnace. By this technique, an expanded and exfoliated graphene
powder was obtained. The prepared graphene powder was evaluated in
the same manner as in Example 1.
Comparative Example 2
[0076] The graphene oxide gel prepared in Synthesis Example 1 was
dried at 100.degree. C. for 24 hours in a heating furnace to give a
graphene oxide powder. The dried graphene oxide powder was
dispersed in NMP so as to be 30 mg/ml, and 100 parts by weight of
the dispersion was mixed with 92 parts by weight of
LiMn.sub.2O.sub.4 as an electrode active material by a planetary
mixer. Further, in the resultant mixture, 1.5 parts by weight of
acetylene black as a conductive agent and 5 parts by weight of
polyvinylidene fluoride as a binder were added and mixed by a
planetary mixer to give an electrode paste. The electrode paste was
applied to aluminum foil (having a thickness of 18 .mu.m) by using
a doctor blade (300 .mu.m), and dried at 170.degree. C. for 5 hours
and then heated at 200.degree. C. for 20 hours in a reducing
atmosphere to give an electrode plate. In the drying and heating
steps of the electrode, graphene oxide was reduced at the same
time, and a graphene powder was generated. When the electrode plate
was measured in accordance with the procedures in Measurement
Example 6, the discharge capacity was 15 mAh/g.
[0077] The electrode components were exfoliated from the electrode
plate, PVDF was washed away by using NMP, the electrode active
material was dissolved with an acid, and the resultant product was
dried and a graphene powder was taken out. This graphene powder was
evaluated in the same manner as in Example 1.
Comparative Example 3
[0078] The graphene oxide gel prepared in Synthesis Example 1 was
diluted with ion-exchange water to a concentration of 5 mg/ml, and
treated in an ultrasonic bath to give a homogeneously dispersed
graphene oxide dispersion. In 200 ml of the graphene oxide
dispersion, 0.5 g of dopamine hydrochloride and 3 g of sodium
dithionite as a reducing agent were placed. The resultant mixture
was reacted at a reduction reaction temperature of 40.degree. C.
for a reduction reaction time of 30 minutes using a mechanical
stirrer. A process in which the obtained graphene dispersion is
filtered, the filtered material is dispersed again in 100 ml of
water, and the dispersion is filtered was repeated twice, and the
filtered material was washed. After the washing, the resultant
product was subjected to vacuum drying at 120.degree. C. for 2
hours to give a graphene powder. The prepared graphene powder was
evaluated in the same manner as in Example 1.
Comparative Example 4
[0079] The treatment was performed in the same manner as in Example
1 except that the drying method of graphene powder was changed from
freeze drying to drying at 100.degree. C. and normal pressure for 6
hours.
[0080] The preparation conditions of the graphene powder in
examples and comparative examples as described above, and various
evaluation results of the obtained graphene are shown in Table
1.
TABLE-US-00001 TABLE 1 Preparation of graphene oxide Graphene
evaluation dispersion Reduction step Specific Graphene Stirring
step Graphene Surface Powder Discharge oxide Stirring oxide
Reducing Reaction Reaction Drying area O/C N/C Raman resistivity
capacity Yield Diluent concentration Volume Dispersant conditions
concentration agent temperature time step (m.sup.2/g) ratio ratio
I.sub.D/I.sub.G (.OMEGA. .times. cm) (mAh/g) value Example 1 Ion-
30 mg/mL 20 ml Dopamine FILMIX 5 mg/mL Sodium 40.degree. C. 1 hour
Freeze 180 0.12 0.013 1.55 0.042 90 10 Pa exchange hydrochloride
Rotational dithionite drying water 0.3 g speed 40 m/s 0.3 g
Treatment time 60 s Example 2 Ion- 30 mg/mL 20 ml Dopamine FILMIX 5
mg/mL Sodium 40.degree. C. 1 hour Freeze 158 0.12 0.013 1.54 0.043
87 8 Pa exchange hydrochloride Rotational dithionite drying water
0.3 g speed 40 m/s 0.3 g Treatment time 30 s Example 3 Ion- 30
mg/mL 20 ml Dopamine FILMIX 5 mg/mL Sodium 40.degree. C. 1 hour
Freeze 135 0.12 0.013 1.55 0.042 88 7 Pa exchange hydrochloride
Rotational dithionite drying water 0.3 g speed 30 m/s 0.3 g
Treatment time 60 s Example 4 Ion- 10 mg/mL 20 ml Dopamine FILMIX 5
mg/mL Sodium 40.degree. C. 1 hour Freeze 114 0.12 0.013 1.54 0.045
85 7 Pa exchange hydrochloride Rotational dithionite drying water
0.1 g speed 40 m/s 0.3 g Treatment time 60 s Example 5 Ion- 30
mg/mL 20 ml Dopamine FILMIX 5 mg/mL Sodium 40.degree. C. 1 hour
Freeze 176 0.10 0.012 1.41 0.034 82 8 Pa exchange hydrochloride
Rotational dithionite drying water 0.3 g speed 40 m/s 0.3 g
Treatment time 60 s Example 6 Ion- 30 mg/mL 20 ml Dopamine FILMIX 5
mg/mL Sodium 40.degree. C. 1 hour Freeze 172 0.13 0.011 1.79 0.046
83 7 Pa exchange hydrochloride Rotational dithionite drying water
0.3 g speed 40 m/s 0.3 g Treatment time 60 s Example 7 Ion- 30
mg/mL 20 ml Dopamine FILMIX 5 mg/mL Sodium 40.degree. C. 1 hour
Freeze 90 0.12 0.007 1.21 0.038 74 5 Pa exchange hydrochloride
Rotational dithionite drying water 0.3 g speed 40 m/s 0.3 g
(+dilution Treatment after drying) time 60 s Example 1 Ion- 30
mg/mL 20 ml Dopamine FILMIX 5 mg/mL Sodium 40.degree. C. 1 hour
Vacuum 106 0.12 0.015 1.52 0.041. 76 7 Pa exchange hydrochloride
Rotational dithionite drying water 0.3 g speed 40 m/s 0.3 g
Treatment time 60 s Comparative Dried Powder -- -- Reduction 1500
0.07 0 0.81 0.21 12 56Pa Example 1 by thermal expansion Comparative
NMP 30 mg/mL -- -- Heating 450 0.07 0 0.82 0.23 15 42Pa Example 2
reduction Comparative Ion- 5 mg/mL 200 ml Dopamine Stirrer 5 mg/mL
Sodium 40.degree. C. 30 min Freeze 68 0.12 0.011 1.45 0.043 31 7 Pa
Example 3 exchange hydrochloride stirring dithionite drying water
0.5 g 3 g Comparative Ion- 30 mg/mL 20 ml Dopamine FILMIX 5 mg/mL
Sodium 40.degree. C. 1 hour Heat 42 0.12 0.013 1.43 0.042 21 6 Pa
Example 4 exchange hydrochloride Rotational dithionite drying water
0.3 g speed 40 m/s 0.3 g 100.degree. C. Treatment 6 hours time 60
s
* * * * *